A New Post-Harvest Process of Upgrading Coffea canephora (Robusta) Coffee Beans ^†

Abstract

There is a lack of innovation and process development in the post-harvest processing of coffee. The prevailing methods, such as wet washing, semi-dry/honey and natural drying, are still used to improve the sensory quality of coffee. Little attention has been paid to mitigating natural resource issues such as the availability of clean water and wastewater pollution. The aim of this research was to develop a simple process for upgrading Coffea canephora var. Robusta coffee with low water consumption and low effluent pollution. A controlled fermentation process was developed with optimal standard parameters. Process water requirements were reduced by 80% and effluent emissions were reduced by the same factor. A single-origin Robusta coffee drink was evaluated by sensory experts. The sensory profiles and roasting profiles were also evaluated. This innovative process resulted in the development of a pleasantly balanced single-origin Robusta coffee (not blended with other coffees) with a full body and rich flavour profile.

1. Introduction

Currently, coffee farmers are looking for cheap, simple, reliable and manageable post-harvest processing methods that use minimal amounts of process water [1]. Processes that improve coffee beans without negatively impacting natural resources and the environment are rare. Robusta coffee beans (Coffea canephora) as a single-origin cup of coffee is not highly popular among coffee drinkers.

Wet washing (semi-washed and fully washed) and honey (aerobic and anaerobic) are the standard processes used to upgrade coffee beans. These processes use whole coffee cherries, which contribute to high levels of contaminants in wastewater and are difficult to remove [2,3]. The wet-washing process has some drawbacks, such as being time-consuming, requiring high water consumption, resulting in excessive levels of organic pollutants, and uncontrolled fermentation on farms, resulting in a lack of predictability of coffee quality [4]. Countries are constantly debating environmental pollution, but the problem is still growing at an alarming rate. One of the sources of pollution is the wet processing of coffee, which generates huge amounts of wastewater. Taking this into account, various scientists around the world have been struggling to find an efficient and effective method of treatment [5]. Sources of pollution in coffee bean processing are exocarp, mesocarp and pectin. The best way to prevent contamination is to avoid it. Previous studies have often overlooked the significant costs associated with clean water usage and wastewater treatment in coffee processing [6]. The challenge of managing the water used to wash coffee residues is great and it is the bottleneck in addressing the problem of pollution and wastewater for increased sustainability [7].

Other researchers have worked to simplify the methods and optimise the post-fermentation process, which is crucial to make the methods easily accessible to producers and to produce defect-free coffee beans [8]. In addition to pre-harvest management, there are post-harvest practices that should meet consumer demands for final beverage quality [9]. The two main options for cleaning wastewater from wet washing are chemical or biological, and these require expertise that coffee farmers do not typically have, nor can they afford to set up conventional treatment plants.

The aim of this research was to develop a simple process for upgrading Robusta coffee with low water consumption and low wastewater pollution. The research project has two objectives: (i) to reduce the need for large volumes of processing water and avoid the main sources of wastewater pollution, and (ii) to improve the sensory qualities of the Robusta coffee drink.

2. Materials and Methods

The process developed eliminates the source of contamination by using hulled, sun-dried Robusta beans (Coffea canephora var. Robusta) as the raw material. The raw material is treated using standardised parameters such as temperature and duration. The process is carried out in batches. A study of microorganisms on the surface of coffee beans was carried out. The developed method works with sun-dried Robusta coffee beans, which are at the same stage as beans normally ready for the market. Semi-dry and wet washing have 6–7 steps and the developed method uses only 3 steps including cherry drying and hulling [10], the technological innovation of the developed process. This includes a 2-stage fermentation process. Catabolism and fermentation begin immediately after the cherries are harvested. This is the first fermentation process that produces many flavour compounds, while the second step, developed by other researchers [11], uses specific enzymes from fermentation microorganisms to digest carbohydrates and produce desirable flavour molecules. Instead of adding expensive specific enzymes, the new process uses the natural microorganisms found on the beans. Enzymes are not readily available to coffee growers and processors.

Different batches of the processed Robusta coffee beans were evaluated as a single cup by different coffee experts, such as Coffee Mind (Copenhagen, Denmark), including Q-Robusta graders at the Coffee Quality Institute (CQI, Zurich, Switzerland) and a barista at Komma Kaffe (Copenhagen, Denmark). Roasting profiles developed by Coffee Consulate (Mannheim, Germany) were used. Sensory profiles were evaluated by Innova Consult (Aarhus, Denmark). A microbial study was conducted to determine the presence of microorganisms in sun-dried coffee beans and their potential use in the fermentation process. The study revealed the presence of yeasts, bacteria, and fungi in the beans. The other purpose was to find out if there were any pathogenic microorganisms. The analysis method used was Targeted Metagenomics via Genetic Identification by Next Generation Sequencing (NGS), which characterises the microflora (Total Targeted Metagenomic Analyses (TMG)) of raw and fermented Robusta coffee beans. This was carried out by ISI Food Protection (Aarhus, Denmark).

3. Results

3.1. Sensory Evaluation Results

The sensory evaluation conducted by Innova Consult (Aarhus, Denmark) yielded the following results for two samples.

Sample A: Roasted Robusta coffee beans

Evaluation of freshly ground beans:

Scent: Coffee, toast, a little dark chocolate, dark notes, hint of ash, a little toasted nut, a little wood, nut, hint of malt and hint of tobacco.

Evaluation of the brewed coffee:

Scent during brewing: Coffee, Arabica, rounded, roasted, a little sweetness and a little nut.

Scent after brewing: Fresh, coffee, hint of wood/cedar, hint of hay/coumarin, toasted bread, toasted nut, hint of earth and hint of green pepper.

Flavour: Coffee, slightly bitter, fresh, a little dark chocolate, nuanced, hint of sweet/brown sugar and intense but rounded.

Mouthfeel: Long aftertaste and mouthwatering (when you drink water right afterwards, the water tastes sweet).

Sample B: Roasted Robusta coffee beans

Evaluation of freshly ground beans:

Scent: Coffee, a little ash, rounded, a little dark chocolate, toasted bread, toasted hazelnut, hint of wood, dark notes and hint of sweetness/brown sugar.

Evaluation of the brewed coffee:

Scent during brewing: Hint of green pepper, coffee, Arabica, roasted and a little Robusta.

After brewing: Coffee, toast, a little ash, a little Robusta, a little wood, a little earthy, intense, a little burnt sugar, complex and a little green pepper.

Flavour: Acidic, coffee, a little bittersweet, a little bitter, a little wood, a little Robusta, a little earth, intense, complex, a little dark chocolate and a little burnt sugar.

Mouthfeel: Long aftertaste and mouthwatering.

Other sensory profile evaluations:

Coffee Consulate provided sensory profiles that indicate a long mouthfeel, good body, sweetness, and little bitterness. At CQI, the single origin scored 85.58 points distributed on the flavour radar as (a) 8.08 for fragrance/aroma, (b) 8.00 for flavour, (c) 8.08 for aftertaste, (d) 8.08 for mouthfeel, (e) 8.17 for balance, (f) 8.17 for bitter/sweet, (g) 8.17 for salt/acid, (h) 10 for uniformity, (i) 10 for clean cup; (j) 8.83 overall.

3.2. Water Needs and Mucilage Content in Wastewater

The process water requirement for the process was 3 to 4 m³ to process 1 m³ of coffee beans, compared to a wet-wash water requirement of 14 to 20 m³ per tonne [12]. The mucilage content in the effluent was only 0.7% w/w as measured by Eurofins/Steins (Vejen, Denmark).

3.3. Microbial and Aflatoxin Content Analyses

ISI Food Protection found more than 30 microorganisms, including Leuconostoc mesenteroides, Lactobacillus helveticus and Aspergillus species (and other fungi), on the surface of raw and fermented Robusta beans; Aspergillus fungi are common in cereals and coffee beans and can produce aflatoxins during fermentation [13]. Aflatoxins are carcinogenic. It was therefore necessary to test for the presence of aflatoxins B1, B2, G1 and G2 in our ferments. The Eurofins/Steins laboratories were able to measure concentrations of <0.1 µg/kg in our coffee samples. According to the U.S. Food and Drug Administration (FDA), aflatoxin levels in human food can range from 4 to 30 µg/kg, depending on the importing country, while the European Union limit is 4 µg/kg. Figure 1 shows Robusta coffee beans at different stages of processing.

4. Discussion

Standard sun drying tends to produce low-quality coffee beans. The developed process can be used to improve the quality of Robusta coffee. The standardised parameters prevent the risk of over-fermentation. The smell and taste of over-fermented coffee beans cannot be removed even after roasting. Roasters avoid processing these coffee beans as they can contaminate their expensive machinery.

The new processing method effectively upgrades sun-dried Robusta coffee beans and represents a viable option for single-origin coffee products in the marketplace. Further research and wider market trials could help to establish its commercial viability. Further academic work should also be undertaken to compare roasting conditions for different concentrations of coffee safety-related compounds such as acrylamide compounds. A comparison of the coffee sensory qualities of samples from the developed method with those from the wet-washed post-harvest processing method should also be undertaken. This will help to define the Robusta coffee in relation to the other types of coffee that have been processed using the traditional methods.

The practical implications of the developed method are that farmers can now obtain a better price for their improved coffee beans. The process is stationary and inexpensive to install and operate. Operating costs include drying, sorting the beans, cupping and packaging the finished product. The process reduces water consumption by 75–80% compared to wet washing and reduces effluent by the same factor. Only solar energy is needed. The low demand for clean water is welcome in view of the increasing frequency of droughts and floods due to climate change. The small amount of slightly polluted wastewater can be biologically treated by the root zone and reused. In Ethiopia, most coffee processing plants produce large amounts of wastewater with high concentrations of pollutants which is discharged untreated or partially treated and flows directly into water bodies. The physicochemical parameters of these raw effluents exceed the national and international discharge limits [14], the problem is also addressed by Genanaw et al. [15].

Wet washing and honey processing are seasonal activities and workers need to find other sources of income until the next harvest season. The harvest season for coffee cherry beans generally lasts from June to December and can vary from region to region, and this is when the normal post-harvest processing activities begin. Low-quality Arabicas can also be upgraded using this method. The composition of microorganisms (yeasts, fungi and bacteria) will always vary from region to region, depending mainly on the level of hygiene, soil, water and air fauna and flora.

5. Conclusions

This research reduced the number of steps required in the post-harvest processing of coffee beans and demonstrated that there is no need to add microorganisms or expensive enzymes to aid fermentation. The time required to process coffee beans was drastically reduced. The volumes of process water and wastewater were reduced. Wastewater pollution was reduced by eliminating the source of pollution, namely the cherries, which were excluded in the process. This method offers a better cost–benefit ratio than wet washing. Sensory evaluation showed that Robusta can be served as a pleasant single-origin cup of coffee.

A cupping evaluation of the developed Robusta coffee and some Arabica coffees is one of the next steps to further test the product of the developed process. Baristas’ awareness of this product can help to spread and promote single-origin Robusta in the coffee market. It is hoped that the developed process can be applied to upgrade natural arabica coffee beans as well.